The invention is based on a sensor for determining the concentration of oxidizable constituents in a gas mixture, in particular for determining one or several of the gases NOx, CO, H2 and preferably unsaturated hydrocarbons, of the generic type as in the main claim. The exhaust gases from Otto and diesel motors, internal combustion engines and combustion plants can contain increased concentrations of oxidizable constituents, in particular NOx, CO, H2 and hydrocarbons, e.g., as a result of malfunctions of components such as an injection valve, or a as a result of an incomplete combustion. Thus, it is necessary to know the concentration of these exhaust-gas constituents to optimize the combustion reactions. A method for determining oxidizable gases is described in the JP-0S 60061654, according to which a stoichiometric conversion with oxygen takes place at a first measuring electrode of platinum metals, and according to which quasi equilibrium states are adjusted for the oxygen balance reaction at one or several additional metallic measuring electrodes with reduced catalytic activity. The Nernst voltages E1 and E2 are measured between the measuring electrodes and a reference electrode that is subjected to a reference gas with constant oxygen partial pressure, and their difference based on calibration curves is used to calculate the concentration of the gas constituents.